Why wings?

How have some animals evolved wings?

When learning about evolution, someone might wonder about how irreducibly complex parts of animals, such as wings, have evolved.  A wing only works when there is an entire functioning wing. So this brings up the question, how does an animal without wings evolve into an animal with wings? The most reasonable explanation would be that wings began evolving with a slightly different purpose. 

Theropods that lived in the trees would get around by jumping from branch to branch. If they missed the jump, they could fall out of the tree and die. These theropods evolved into birds. According to discovermagazine.com, birds and theropods both "Have three bones that appear to have evolved from the digits on a common five-fingered ancestor." A theropod with a small flap of skin between its fingers would have a slightly larger surface area when jumping between branches and that would decrease the chance of it falling. Through generations, that skin flap became larger and eventually evolved into a functioning wing where the animal could flap it's wings and fly away.

If I were to do further research, I would want to look into birds that don't fly, like penguins or emus. If they can't fly, what is the purpose of having wings?

Sources:

http://blogs.discovermagazine.com/80beats/2009/06/18/how-dinosaur-feet-evolved-into-bird-wings-new-fossil-provides-clues/

The Blind Watchmaker by Richard Dawkins